There has been known an illumination system wherein an AC power source is connected to a triac, which is a bidirectional triode thyristor and performs on/off control of the AC power source. In the illumination system, a light emitting diode (LED) lamp is on and off controlled by the on/off control of the triac (see, e.g., Japanese Patent Application Publication No. H05-066718). The above-described example includes a diode bridge circuit which rectifies the AC power from the triac performing the on/off control of an AC power source to thereby provide the DC power which serves to light the LED lamp.
The triac has a “self-sustaining function”. That is, once triggered, the triac continues to conduct until the current therethrough falls below a so-called holding current.
When performing the on/off control of the above-mentioned triac, a so-called pulse trigger method is normally employed in which a pulse is inputted to a gate terminal of the triac only when the triac is switched to a conducting state. In this pulse trigger method, however, the current flowing through the triac may drop below the holding current due to a noise overlapped on a power line of a commercial power source and thus the triac may be suddenly switched to a non-conducting state (turned off). Further, a repetition of this phenomenon may cause flickering during lighting a lighting load. Especially, when an LED is employed as the lighting load and is dimmed as in the foregoing conventional example, a noise may be easily overlapped on the power line of the commercial power source as compared to when an incandescent bulb is used as the lighting load, and thus the above-mentioned problem may easily occur.